Electronic storage tubes having target structures of the coplanar grid type are presently in use and are extremely advantageous for use in a wide variety of applications in which it is desired to generate an image of data, pictures and other like material, store the image for substantially long periods of time and repeatedly read out the image, for example, for display purposes in cathode ray tube display devices, wherein repeated read out and display operations do not impair the stored image.
Electronic storage tubes of the type described hereinabove typically employ target structures comprising a layer of conductive material such as conducting silicon and a coplanar grid structure affixed thereto and which, in turn, is usually comprised of a striped pattern of elongated strips of a suitable insulation material such as, for example, a silicon dioxide layer which is arranged in such a fashion upon the conducting silicon as to produce a striped pattern, wherein every pair of adjacent insulation strips are separated by an exposed surface area of conducting silicon.
The insulating grid structure serves as a storage means for storing an electrical charge pattern to develop a surface potential upon the target which pattern represents a stored image.
The storage pattern is developed by scanning the target with an electron beam which sweeps across the target. Simultaneously therewith, the electronic storage tube electron gun control grid has a modulating voltage applied thereto which represents the image or data to be stored and which is employed to modulate the electron beam as it sweeps the target.
Prior to the writing mode, the target is erased, that is conditioned preparatory to image storage by sweeping the target with an unmodulated electron beam (of substantially constant current density) to create a uniform negative charge pattern on the insulator surface which results in an insulator surface potential which may typically be of the order of ten to twenty volts lower than the target voltage applied to the target. During the write mode the target voltage is typically of the order of 200 to 300 volts. The insulator surface potential, although 10 to 20 volts lower than the target voltage, is still nevertheless at a high voltage level causing the electron beam to strike the target member at a velocity which causes the electrons on the grid surface to be "knocked off" in quantities greater than those electrons which land and are retained upon the surface. This "secondary emission" effect drives the surface potential more positive with the degree of increase in the positive direction being a function of the intensity of the electron beam and its "dwell time" at each point. This operation creates and stores an image having an insulator surface charge pattern and hence a surface potential which is a function of the stored image.
Read-out of the stored image may be performed by first reducing the target voltage to a value such that all points of the insulator surface return to negative values (typical read target voltage values are 5 to 10 volts), and scanning the target with an unmodulated electron beam. The coplanar grid functions in much the same manner as the control grid of a vacuum tube triode which reduces electron current flow to the anode as the control grid is driven more negative relative to the cathode and which increases the electron flow to the anode as the control grid goes more positive relative to the cathode. In a like manner, those locations on the coplanar grid surface which are at or slightly below cathode potential during the read operation permit maximum target current, while those points of the surface potential pattern which are increasingly more negative than the cathode potential conversely reduce target current until the point is reached where the negative surface potential is sufficient to prevent any electrons from striking the exposed conducting silicon in those regions which are immediately adjacent the most negative surface potential locations. Typically, for a target construction where the exposed conducting silicon area is approximately equal to the insulator surface area, this current cut-off occurs for an insulator surface potential (.phi..sub.i) equal and opposite to the target voltage (V.sub.T) (that is I.sub.TARGET = 0 when .phi..sub.i .apprxeq.-V.sub.T). Typical values are V.sub.T = +10 volts for which .phi..sub.i = -10 volts will stop all current flow. Preferably, after writing, all points of the coplanar grid surface are maintained below the cathode potential to prevent electrons from the electron beam from striking the grid surface, causing no impairment of the stored image so that the stored image can be repeatedly read out many times without suffering degradation in the resolution and quality of the image.
Careful observation of the electronic storage tubes of the types described hereinabove has shown that image fading does in fact occur. Experimentation undertaken by this inventor has shown that in addition to the well known fading mechanism of gas ion discharge of the insulator surface charge, other effects, such as ionizing radiation induced conductivity in the insulating grid play a significant role.